A system, for example, described in Japanese Patent Publication No. Hei 7-27868 is known as a horizontal CVD system for the growth of a compound semiconductor thin film on the surface of a substrate placed in a reactor by introducing a source gas thereto.
The CVD system described in the above official gazette has a cylindrical reactor oriented such that the axis thereof may be horizontal and a susceptor (mount) for retaining a substrate, disposed on the bottom of the reactor. A separator is disposed in the reactor on the upstream side of the substrate mounting section to be parallel to the substrate surface so as to define two parallel passages above and below the separator, i.e. a lower passage on the substrate mounting section side and an upper passage on the counter substrate mounting section side. The reactor has an exhaust pipe on the downstream side of the substrate mounting section. A flow channel for smoothing the gas flow is provided on the upstream side of the susceptor in the reactor, and a heating RF coil is disposed to surround the reactor at the substrate mounting section.
In the CVD system described above, a CVD gas formed by diluting a source gas with a diluent gas is introduced to the upper passage in the state where the substrate is mounted on the susceptor and is heated by the RF coil, and a carrier gas containing no source gas is introduced to the lower passage to grow a deposition film. The carrier gas may be the same diluent gas as contained in the CVD gas or such gas containing a small amount of volatilization preventive gas.
The CVD gas and the carrier gas supplied to the upper passage and the lower passage respectively flow through the distal end of the separator toward the substrate, and the source gas contained in the CVD gas diffuses into the carrier gas under the mutual diffusion actions of the gases and approaches the surface of the substrate with the concentration thereof in the carrier gas being increased gradually. Then, the source gas diffused into the carrier gas undergoes pyrolysis at a high-temperature section around the substrate and deposited on the substrate surface to grow a deposition film thereon.
In this process, the amount of source gas to be diffused from the CVD gas into the carrier gas and the amount of source gas to be consumed from the carrier gas by deposition onto the substrate surface can be balanced per unit time by suitably adjusting the flow velocity of the carrier gas and that of the CVD gas and the concentration of the source gas in the CVD gas, and thus the source gas concentration of the carrier gas flowing over the substrate surface can be allowed to be of uniform distribution in the flow direction, growing a deposition film having a uniform thickness.
As described above, the horizontal CVD system can give a substrate on which a film having a uniform film thickness is formed by introducing a CVD gas and a carrier gas separately in two-layer flows by employing the separator to allow the source gas in the CVD gas to diffuse into the carrier gas and to be carried onto the substrate surface, and also the system can reduce the amount of detrimental deposition on others than the substrate, since the concentration of the source gas flowing over the substrate surface is reduced by the presence of the carrier gas.
However, when a deposition film of two or more components is formed employing such horizontal CVD system, those elements having high volatility among other component elements evaporate from the deposition film, so that the resulting deposition film can contain defects. In order to prevent such defects from occurring, a countermeasure has been conventionally taken, for example, to supply a source gas containing a highly volatile element (volatile gas) in an amount corresponding to or higher than the equilibrium partial pressure of the volatile gas, but it resulted in increased cost due to reduction in feedstock utilization efficiency.
Meanwhile, it is necessary to adjust the gas flow velocity to an optimum condition in order to allow a desired reaction product to be deposited on the substrate surface. However, the control of gas flow velocity requires delicate adjustments in terms of feedstock utilization efficiency, impurity doping essential for producing devices, etc. Thus, it has been difficult in the conventional horizontal CVD system to form thin films for devices having complicated structures satisfying all the requirements for film thickness, composition and doping uniformity.
More specifically, it has been conventionally carried out to adjust the source gas concentration so as to prevent the volatile component from volatilizing or to adjust the gas flow velocity so as to allow the zone on which the reaction product is to be deposited to coincide with the substrate surface. However, the conditions for obtaining a uniform film thickness and the optimum conditions for the source gas concentration do not necessarily agree with the conditions for obtaining excellent film properties. Further, since the optimum gas flow velocity conditions for obtaining film thickness uniformity do not necessarily agree with those for obtaining doping uniformity, the method of adjusting gas flow velocity is not suitable for depositing a thin film having a complicated multilayer structure for devices etc.
Furthermore, in the conventional system, since decomposition products and the like are deposited around the substrate, the reactor must be cleaned frequently.
Further, the CVD system having the conventional separator described above forms thin films having defects due to particles and lowers deposition speed, uniformity of the thin films and reproductivity depending on operating conditions, for example, deposition temperature, the kind of source gas, conditions of gas flow velocity, etc. These problems have arisen frequently particularly when there is a great difference between the flow velocities of the gases in the respective layers defined by the separator.
Under such circumstances, the present inventors made extensive studies to find that turbulence of gas flow occurring in the vicinity of the distal end portion of the separator can be causative of these problems. More specifically, in this CVD system, since no consideration is given to turbulence of gas flow occurring in the vicinity of the distal end portion of the separator, eddy currents can occur in the gas flow at the distal end portion of the separator depending on the balance between the gas flow velocities in the respective layers defined by the separator. If an eddy current occurs at the distal end portion of the separator, a feedstock involved in the eddy current circulates along it to dwell there for a long time, and the temperature of the gas is increased by radiation from a heated substrate to cause pyrolysis of the gas and formation of particles. Thus, the particles formed on the upstream side of the substrate if brought onto the substrate can be causative of crystal defects to lower quality of the resulting film. Further, the particles serve as growth nuclei in the vapor phase to consume feedstocks present around them, leading to reduction in feedstock utilization efficiency. Further, when the source gases supplied to the respective layers are highly reactive with each other, the occurrence of eddy currents promotes synthesis of a reaction product by means of vapor phase reaction to cause deterioration of film quality and lowering in feedstock utilization efficiency.
As described above, in the CVD system having the conventional separator, the gas flow velocity in each layer defined by the separator is limited to a very small range, so that it is sometimes difficult to set gas flow velocity conditions most suitable for thin film deposition.
A first objective of the present invention is to provide a CVD system and a CVD process which can form an excellent compound semiconductor multilayer thin film of two or more components having reduced defects and which enjoy high source gas utilization efficiency and increased productivity.
A second objective of the present invention is to provide a CVD system which can secure a wide setting range of gas flow velocity in each layer defined by the separators and which can grow a compound semiconductor thin film of two or more components, having excellent quality and excellent uniformity with high reproductivity without being affected by the gas flow velocity in each layer and without reducing the feedstock utilization efficiency.